A Quantitative Detection Method for Free-Water Concentration in Petroleum Products Based on Sodium Fluorescein

The free water in petroleum products seriously affects their performance; however, there is currently a lack of rapid, accurate, and quantitative methods for free-water detection. Herein, we present a novel detection method for the free-water concentration in petroleum products by means of a composite detecting diaphragm composed of sodium fluorescein nanocrystals. The identification of free water is based on its fluorescent effect, in which sodium fluorescein absorbs light at a wavelength of 492 nm and emits fluorescence at a wavelength of 513 nm under ultraviolet-light excitation. The results show that this method can quantitatively and accurately detect the free-water concentration in a few minutes in jet fuel, lubricating oil, and so on. Furthermore, the morphology and structural changes of sodium fluorescein with and without water were also studied using scanning electron microscopy, X-ray powder diffraction, and Raman and infrared spectroscopy. This study presents deep insight into the fluorescent effect of sodium fluorescein nanocrystals and offers a promising, general approach for determining the free-water concentration in petroleum products.